The invention relates to a process for preparing biuret-functional polyisocyanates from (cyclo)aliphatic diisocyanates and at least one amine or water or a mixture thereof, together if desired with at least one alcohol, as reactants which are mixed with one another in a mixing unit having a high shear action. Biuret-functional (cyclo)aliphatic polyisocyanates are employed, inter alia, in high-grade light-stable and weather-resistant two-component PU coating materials, and in adhesives and dispersions. To prepare biurets, a defined amount of a biuretizing agent, for example water, water donors, amines or ureas, is added to the diisocyanates, and these compounds are reacted at, normally, from 100 to 200xc2x0 C. The excess isocyanate monomer is subsequently separated off by single-stage or multistage distillation. A good review of the various options for preparing biurets is given, inter alia, by DE-A 34 03 277, EP-A 0 716 080, DE 195 25 474.0, and by a review article in J. prakt. Chem. 336 (1994), 185-200.
The literature has already disclosed processes for the direct preparation of biurets from isocyanates and amines.
For example, DE-A 22 61 065 describes, inter alia (Example 16), the reaction of excess amounts of 1,6-hexamethylene diisocyanate (HDI) with 1,6hexamethylenediamine (HDA). According to this Example, the reactants are stirred at 180xc2x0 C. for 12 hours. This long period of subsequent heating at a high temperature not only is extremely uneconomic but leads, especially under industrial production conditions, to a discoloration of the product, so that its use in lightfast coating materials is greatly restricted. Reworking of the above-mentioned Example 16 gave a highly viscous biuret with a considerable proportion of solids. It is therefore not possible to use the process described there to obtain a biuret polyisocyanate which is free from monomeric starting diisocyanate and is also completely free from insoluble, gel-like by-products.
DE-A 26 09 995 attempts to circumvent the disadvantages of the formation of solids in DE-A 22 61 065 by introducing the amines in gaseous form, at from 100 to 250xc2x0 C., into the initial charge of diisocyanate. As a result of the in all cases high dilution of the diamines introduced in gaseous form, instances of precipitation of polyurea occur to a much less extent, although even here it is impossible to avoid completely the formation of urea agglomerates, since they form by local overconcentration of the reactants on the nozzle itself, thereby blocking it. In addition, owing to the use of gaseous diamines, large volumes are required to perform this process on the industrial scale, thereby making it difficult to control the reaction conditions.
EP-B 0 003 505 attempts to circumvent the problem of nozzle blockage by injecting the amine into the initial charge of isocyanate through a smooth-jet nozzle under high pressure (2xc2x7105 to 1xc2x7108 Pa) at from xe2x88x9220 to 250xc2x0 C.
In the process described in EP-B 0 277 353, the formation of solids is avoided by combining the amines, with or without the addition of water or alcohols, with the isocyanate through nozzles in a mixing chamber at above 250xc2x0 C. and at pressures of up to 1xc2x7107 Pa. After-reaction to adjust the molecular weight distribution takes place subsequently in a stirred vessel at from 80 to 220xc2x0 C. A process of this kind is technologically expensive owing to the high pressures and the nozzle technology. In the downstream reactors, however, the formation of solids may still be a problem. The high temperatures of more than 250xc2x0 C. are already close to the decomposition point of HDI, for example, so that it is not possible to rule out thermal damage to the product, which is manifest in a dark coloration.
As is evident from the above, using the processes described to date it is possible only with extreme difficulty to prepare virtually colorless, biuret-functional polyisocyanates without the concomitant formation of large amounts of disruptive solids. Since biuret-functional diisocyanates are preferentially employed in the clearcoat sector, discoloration and a high solids content in these isocyanates are disadvantageous. Many of the above-described processes, furthermore, can be operated under industrial production conditions only at great technological expense.
It is an object of the present invention, therefore, to provide a process for preparing biuret-functional polyisocyanates which is free from the disadvantages described above.
We have found that this object is achieved, surprisingly, in that biuret-functional polyisocyanates with commercial viscosities of from 3000 to 10,000 mPas and good color numbers of 500 Hz or less, preferably 300 Hz or less, in particular 100 or less, can be produced under moderate conditions by mixing an aliphatic, cycloaliphatic or araliphatic diisocyanate and at least one amine or water, or a mixture thereof and, if desired, an alcohol and bringing them into contact with one another in a mixing unit having a high shear action.
The present invention accordingly provides a process for preparing biuret-functional polyisocyanates from at least one aliphatic, cycloaliphatic or araliphatic diisocyanate or a mixture of two or more thereof and at least one amine or water or a mixture of two or more thereof as reactants, which comprises mixing the reactants with one another in a mixing unit having a high shear action.
Furthermore, the present invention relates to a biuret-functional polyisocyanate preparable by a process for preparing a biuret-functional polyisocyanate from at least one aliphatic or cycloaliphatic or araliphatic diisocyanate or a mixture of two or more thereof and at least one amine or water or a mixture of water and at least one amine as reactants, in which the reactants are mixed with one another in a mixing unit having a high shear action and are reacted together.
The term xe2x80x9cbiuret-functional polyisocyanatexe2x80x9d as used in the context of the present invention relates to polyisocyanates, as defined within the context of the present invention, which as principal component comprise molecules with the formula 
and homologous isomers thereof, where X is an aliphatic, cycloaliphatic or araliphatic alkylene group.
Appropriate starting products for the preparation of the biuret-functional polyisocyanates are the aliphatic, cycloaliphatic or araliphatic diisocyanates which are known per se, individually or in mixtures of two or more thereof. These are preferably alkylene diisocyanates having 4 to 12 carbon atoms in the alkylene radical, such as 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate (HDI), 1,12-dodecamethylene diisocyanate, 2ethyltetramethylene 1,4-diisocyanate, 2-methylpentamethylene 1,5-diisocyanate, 2,2,4-trimethylhexamethylene 1,6-diisocyanate,2,4,4-trimethylhexamethylene 1,6-diisocyanate and lysine alkyl ester diisocyanates; cycloaliphatic diisocyanates, such as cyclohexane 1,3- and 1,4-diisocyanate, isophorone diisocyanate (IPDI) and bis(4-isocyanatocyclohexyl)methane, 2,5- and 2,6-diisocyanatomethylnorbornane, or araliphatic diisocyanates, such as xylylene diisocyanate and tetramethylxylylene diisocyanate, with particular preference being given to the use of 2-butyl-2-ethylpentamethylene diisocyanate, 2methylpentamethylene diisocyanate, IPDI, HDI and bis(4-isocyanatocyclohexyl)methane.
Further starting materials for the process according to the invention are organic mono- and diamines having aliphatically and/or cycloaliphatically attached primary or secondary amino groups. These include, for example, aliphatic or cycloaliphatic monoamines of the formula Rxe2x80x94NH2 where R is an aliphatic hydrocarbon radical of 1 to 12 carbon atoms or a cycloaliphatic hydrocarbon radical of 5 to 7 carbon atoms, such as, for example, methylamine, n-butylamine, n-dodecylamine, cyclopentylamine, cyclohexylamine or cycloheptylamine. Ammonia can also be considered.
Mention may also be made of diamines, containing primary amino groups, of the formula Rxe2x80x2(NH2)2 where Rxe2x80x2 is an aliphatic hydrocarbon radical of 2 to 12 carbon atoms or a cycloaliphatic hydrocarbon radical of 4 to 17 carbon atoms. Examples which may be mentioned here are ethylenediamine, 1,2- and 1,3-propylenediamine, 1,4-diaminobutane, 2,2dimethythylpropane-1,3-diamine, 1,6-hexamethylenediamine, 2,5-dimethylhexane-2,5-diamine, 2,2,4-trimethylhexane-1,6-diamine, 1,8-diaminooctane, 1,10-diaminodecane, 1,11-undecanediamine, 1,1 2-dodecanediamine, 1methyl-4-aminoisopropylcyclohexyl- 1-amine, 3-aminomethyl-3,5,5-trimethylcyclohexyl-1-amine, 1,2-bis(aminomethyl)cyclobutane, 1,2- and 1,4-diaminocyclohexane, 1,2- and 1,4-,1,5- and 1,8-diaminodecalin, 1-methyl-4-aminoisopropylcyclohexyl-1-amine, 4,4xe2x80x2-diaminobicyclohexyl, bis(4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminodicyclohexylmethane, 1,2-bis(4-aminocyclohexyl)-ethane, 3,3xe2x80x2, 5,5xe2x80x2-tetramethylbis(4-aminocyclohexyl)methane and -propane, and isophoronediamine, and mixtures of two or more thereof.
Particularly preferred amines employed are hexamethylenediamine, isophoronediamine and bis(4-aminocyclohexyl)-methane.
In a particularly preferred embodiment of the present invention, the diisocyanate employed is hexamethylene diisocyanate and the amine employed is hexamethylenediamine.
In the process according to the invention the amines can also be employed as a blend with water and/or aliphatic alcohols, in which case tertiary alcohols in particular are employed. Examples which may be mentioned are 1,4-dihydroxybutane, neopentylglycol, trimethylolpropane, glycerol and tert-butanol, the latter being particularly preferred. It is also possible to employ water alone.
When carrying out the novel process, the starting diisocyanates and the amines, water and mixtures thereof, with or without an alcohol, are reacted in a proportion such that the ratio of equivalents of isocyanate groups to the isocyanato-reactive groups is at least 2:1, preferably from 4:1 to 25:1 and, in particular, from 7:1 to 20:1, the primary amino groups entering the calculation as difunctional groups. These reactants are introduced, preferably continuously, into the reactor in the above-mentioned proportion.